The present invention relates to a smokable synthetic food-packaging film and, more particularly, to a smokable synthetic food-packaging film which shows a smoke-permeability and an oxygen-barrier property, and which also has excellent smokability and preserving effect without secondary packaging in plastic bags.
Hitherto, the packages for foodstuffs such as prepared meat which is to be smoked is mainly conducted by primary packaging and secondary packaging. For primary packaging, a packaging material is selected so that it is suitable for forming and maintaining the shape of foodstuffs which are packed directly into the packaging material, and for rapidly permeating smoke components during the smoking process. As examples of packaging materials used for primary packaging, casings made from animal intestines such as those of cows, pigs, or sheep, and cellulose casings made of viscose film may be mentioned. These smokable primary packaging materials, however, lack gas-barrier properties and are unsuitable as packaging materials for ordinary food-storage. Therefore, it is necessary to package the foodstuffs again in a film which has a gas-barrier property (secondary packaging).
Other known smokable primary packaging materials are films of natural polymer such as collagens, chitins and polysaccharides, or paper which is impregnated with a polyvinyl alcohol resin ((Japanese Patent Publication No. 47-43198 (1972)), paper impregnated with an ethylene - vinyl alcohol copolymer containing a plasticizer (Japanese Patent Application Laid-Open (KOKAI) No. 52-57347 (1977)), and polyester copolymer films containing polyalkylene oxide (Japanese Patent Publication No. 59-117530 (1984)). These smokable primary packaging materials also have insufficient gas-barrier property, and thus necessitate a secondary packaging for the preservation of the food. Although films which are mechanically perforated or porous films have been proposed in order to improve the smoking properties (British Patent No. 1,397,472), these packaging materials also necessitate secondary packaging. Thus, conventional smokable packaging materials require a secondary packaging process, and as a result there are defects of the increases of the number of manufacturing steps, packaging materials, and packaging machines, and thus increases the manufacturing cost.
As known films having both smoke-permeability and oxygen gas-barrier property, there are films of polyamides such as polycaprolactam and of polymer blends of polyamide and at least one of ionomer resin, modified ethylene-vinyl acetate copolymer and modified polyolefin (European Patent Application No. 0,139,888Al).
European Patent Application No.0,139,888Al suggests that films consisting of polyamides which can absorb at least 3 wt % of their own weight of water up to saturation point, such as, for example, polycaprolactam, polyamino-oenanthic acid amido, polyhexamethylene adipamide and polyhexamethylene sebacamide, and films consisting of polymer blends of polyamides such as those described above, and at least one of ionomer resin, modified ethylene-vinyl acetate copolymer and modified polyolefin, are useful as smokable thermoplastic synthetic casings. However, the only films that are concretely disclosed therein are transparent, colorless shrinkable casings formed of polycaprolactam or polyhexamethylene adipamide. The casings of polycaprolactam and polyhexamethylene adipamide show an insufficient oxygen gas-barrier property, and the casings thereof are therefore inadequate for the prolonged storage periods of foods, in particular, for foodstuffs which are susceptible to oxygen.
Accordingly, it is an object of the present invention to provide a food-packaging film of a thermoplastic resin which has good smokability and excellent oxygen gas-barrier property.
Namely, it is an object of the present invention to provide a smokable food-packaging film which shows such good smoking effect and oxygen gas-barrier property, and can be used to preserve foodstuffs which are susceptible to oxygen for long periods.
As a result of various studies into how to achieve this aim, it has been found by the present inventors that a food package which has smokability and preservability for long periods is obtained without secondary packaging by using a film having at least one layer of a mixture of 45 to 85 wt % polyamide, 10 to 45 wt % copolymer of olefin and vinyl alcohol, and 5 to 30 wt % polyolefin, the layer having the properties of a permeability to 50% concentration of methanol of not less than 200 g/m.sup.2.day.atm at a temperature of 60.degree. C. and a relative humidity of 0% and an oxygen gas-permeability of not more than 50 cc/m.sup.2.day.atm at a temperature of 30.degree. C. and a relative humidity of 60%. The present invention has been attained on the basis of this finding.